PCI Express (PCIe) is a motherboard-level computer bus, which was created in 2004 by Intel, Dell, HP and IBM. PCIe was designed to replace older bus standards, for example PCI, PCI-X and AGB. PCIe specifications are described, for example, in “PCI Express® Base Specification,” Revision 4.0, version 1.0, Oct. 5, 2017, issued by PCI SIG, which is incorporated herein by reference.
The PCIe bus is highly scalable and fully serial, supporting from one to thirty-two lanes, wherein each lane comprises two pairs of differential wires, one pair for each signal direction. PCIe specifications include features like Power Management, Quality of Service (QoS), Hot-Plug/Hot-Swap support, Data Integrity, Error Handling and Bifurcation.
Some buses, PCIe being one example, support bifurcation. Bifurcation is a feature, wherein the bus can be split into several smaller-width buses. Thus, for example, a 16-lane PCIe implementation can be split into two 8-lane buses, four 4-lane buses, eight 2-lane buses or 16 one-lane buses. Bifurcation is described, for example, in section 2.6.2.2. of Intel® Xeon® Processor C5500/C3500 Series Datasheet—Volume 1, February 2010.